This invention relates to a method and an apparatus for tightening a screw joint to a predetermined axial load.
According to the most common joint pretensioning method the joint is tightened to a certain torquelevel. This torque level has been determined experimentally to correspond to a desired tension in the joint. Due to variations in friction, this method suffers from very large deviations in obtained axial load.
A previously known method to avoid this drawback is described in U.S. Pat. No. 3,939,920. According to the described method, the screw joint is tightened to its yield point and the applied torque at this point is registered. This method is based on the fact that at the yield point a certain axial load is obtained in relation to a certain torque. This torque is in turn dependent on the actual friction forces in the joint. From this relationship it is possible to determine the obtained axial load at the yield point by measuring the applied torque. Thereby, the axial load/torque relationship is determined for a certain joint. In order to obtain a desired axial load in a joint according to this method the joint is tightened to its yield point in order to establish the actual load/torque relationship, whereupon the joint is slackened and retightened to a torque level which corresponds to the desired axial load magnitude. This previously known method suffers from two serious sources of error which seriously impair the accuracy of the obtained axial load. One of these is due to the fact that, in the theoretical basis for determining the axial load at the yield point, it has been assumed that the friction forces are of the same magnitude in the thread as beneath the head of the screw or the nut. This is the case in exceptional cases only and, normally, there is obtained deviations in the axial load. The theory is based on the fact that the friction forces in the thread of the joint give rise to a torsion load in the screw which affects the torque obtained at the yield point. The friction forces acting under the head of the screw or/and the nut also result in an augmented torque level but does not influence upon the tension of the screw. Variations in the friction forces beneath the screw head and/or nut therefor cause deviation in the obtained axial load.
The other source of error in this known method relates to the fact that the joint has to be tightened twice. However, at the second tightening, the friction forces in the joint are considerably less than at the first tightening, which means that the torque magnitude which was estimated to give the desired axial load from the first tightening will be too high. Moreover, the variations in the friction reduction are considerable.
In addition, this known method requires a relatively long cycle time and a complicated control system for the nutrunner.
The object of the present invention is to solve this problem, which is accomplished by the invention as it is defined in the claims.